We demonstrate an optimized 2.7 µm Er,Pr:YAP laser in both free-running and Q-switched modes. In free running mode, a maximum energy of 346 mJ is obtained with repetition frequency of 5 Hz. In Q-switched mode, a giant pulse with 63.4 mJ energy and 40 ns pulse width is realized at 5 Hz, which corresponds to an energy extraction efficiency of 47% and a peak power of 1.59 MW. Additionally, the M2 factor and laser spectrum are also measured in two modes. These results indicate that the LGS Q-switched 2.7 µm Er,Pr:YAP laser is a promising candidate for the mid-infrared pulse laser device.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Mid-infrared lasers in the range of 2.7∼3 µm have wide applications in biomedical [1,2], optical parametric oscillator (OPO) , remote sensing, LIDAR, space military and scientific research , etc. Thereinto laser in this region with Q-switched mode is prefer to utilize in the biomedicine field, because the laser is located in the strong water absorption band and the extent of thermal damage to tissue can be reduced by shortening laser pulse duration . Besides, compared with the laser near 2.94 µm and 2.79 µm, the laser near 2.7 µm plays a more important role in laser surgery due to its lower transmission loss in silica fiber [5,6].
Er3+ serves as an active ion that can emit a 2.7~3 µm laser by the transition of 4I11/2 → 4I13/2. However, the transition is restricted commonly by the self-terminating effect due to the lifetime of the laser upper level is far less than that of the lower level. This self-terminating “bottleneck” effect can be restrained by means of co-doping the deactivation ions, such as Pr3+, Eu3+, etc. These deactivation ions usually have energy levels adjacent to level 4I13/2 of Er3+, so it is beneficial to accelerate the particle evacuation rate of lower laser level and realize the population inversion [7,8].
As we know, YAlO3 (YAP) crystal is an attractive candidate as a laser host due to its excellent thermodynamics and mechanical properties, natural birefringence , structural anisotropy and lower phonon energy (570 cm−1) . Furthermore, a linearly polarized emission is easily available from YAP lasers . The Er,Pr:YAP crystal is a promising mid-infrared laser medium, which can combine the abundant fluorescent lines of Er3+and the excellent characteristic of YAP crystal well together. Moreover, in our previous work, the research results indicated that the lower laser level lifetime of the Er,Pr:YAP crystal was greatly reduced by co-doping the deactivation Pr3+ ions .
An important method to gain a short pulse and high peak power laser is electro-optic (EO) Q-switched technique, which has the advantages of high efficiency, fast switching speed, high stability and controllability. Several infrared EO Q-switched crystals for the 2.7∼3 µm wavelength range have been developed, such as La3Ga5SiO14 (LGS), LiNbO3 (LN), etc. The LGS crystal has a higher damage threshold (950 MW∕cm2,1064 nm,10 ns) and lower dielectric constant(ɛ11=18.27) than that of LN (ɛ11=84.6) crystal, which is beneficial to generating higher laser peak power and shorter laser pulse. Until now, LGS has been successfully applied to 2.79 µm Cr,Er:YSGG laser , 2.097 µm Cr,Tm,Ho:YAG laser  and 1.064 µm Nd:YAG laser . Wang et al. used LGS as a Q-switched crystal in a lamp pumped 2.79 µm Cr,Er:YSGG laser and obtained a pulse energy of 216 mJ with a pulse width of 14.36 ns, corresponding to a peak power of 15 MW . However, there is still no literature about LGS Q-switched 2.7 µm Er doped YAP laser can be found.
In recent years, Kawase et.al. have demonstrated a diode pumped Er:YAP laser , a maximum output power of 674 mW was obtained with a slope efficiency of 31%. Furthermore, passively Q-switched diode pumped Er:YAP operation was performed with a monolayer graphene as saturable absorber , the maximum average output power reached 1 W in CW and 503 mW in Q-switched modes, the shortest pulse duration of 460 ns was obtained with pulse energy of 5.1 µJ and peak power of 10 W. There is no doubt that diode pumped lasers have many advantages like compactness, high efficiency and better beam quality. Actually, many studies in recent years have also focused on Q-switched diode-pumped Er-doped lasers, for example, Messner et.al. reported a newly developed Q-switched diode side-pumped Er:YLF solid state laser emitting at 2.81 µm , a peak power of 50 kW with pulse width of 70 ns and pulse energy of 3.5 mJ was realized at a repetition rate of 100 Hz. By contrast, the flashlamp pumped lasers have the defect of low efficiency, serious thermal lens effect and poor laser beam quality. But a high pulse energy and a high peak power with short pulse width can be obtained in the flashlamp pumped Q-switched mode, which still has an important application in the surgery so far.
In this work, we report the laser performance of Er,Pr:YAP crystal for the first time. A flash lamp pumped 2.7 µm Er,Pr:YAP laser in free-running and Q-switched modes is also demonstrated.
2. Experimental setup
The Er,Pr:YAP crystal was grown by the Czochralski (Cz) method. The concentrations of Er3+and Pr3+ in the raw materials are 10 at.%, and 0.02 at.%, respectively. The experiment schematic diagram of the LGS as a Q-switch in Er,Pr:YAP laser is shown in Fig. 1. The Er,Pr:YAP crystal was cut along b-axis and processed into crystal rods with sizes of Φ 4 mm × 100 mm, and the two end faces of the laser crystal rod were coated with an antireflection film near 2.79 µm. A z-cut LGS crystal was processed in a size of 7 mm × 7 mm × 50 mm, the larger horizontal-to-vertical length proportion can decrease the quarter-wave voltage. Moreover, a gold electrode was uniformly coated on the two side faces of LGS crystal, in which electric field was added along the x axis and the laser propagated along z axis of the LGS crystal. LGS crystal has a refractive index of 1.8568 at 2.71 µm and an EO coefficient of γ11=2.3 pm/V. The quarter-wave voltage at 2.71 µm was calculated to be 6.642 kV for the LGS crystal described above. Three 1-mm-thick Al2O3 plates at Brewster angle (59.8°) placement were used as a medium polarizer to acquire polarized light. A xenon lamp with diameter of 5 mm and active length of 80 mm was chosen as a pump source. The xenon flash lamp and crystal rods were both placed into a close-coupled diffusing ceramic cavity and cooled with circulating water, which was maintained at a temperature of 20 °C. A plane parallel resonator with a length of 336 mm was formed by a high reflective mirror (HR) and an output coupling mirror (OC). The HR mirror possesses a high reflectivity at 2.6∼2.9 µm (≥ 99%), the OC mirrors are partially transmitting mirrors with different transmittance of 5%,15%,30%,40% and 70% at 2.7 µm. The laser output energy was measured by an energy meter (Ophir PE50-DIF-C). The laser pulse width was detected by an infrared detector (VIGO System S.A., PVM-10.6) and displayed on a digital oscilloscope (Tektronix MDO3104, 20 Gs/s sampling rates, 1 GHz bandwidth).
3. Result and discussion
3.1 Er,Pr:YAP laser and polarization characteristics
The output pulse energy of Er,Pr:YAP crystal rods versus pumping energy with different OCs operated at 1, 5,10 and 20 Hz are shown in Fig. 2. A maximum laser energy of 346 mJ is obtained with repetition frequency of 5 Hz and OC of 30% transmission. The experiment results show that the output pulse energy increases with the increase of repetition rates under the same pump energy, which may be due to the pumping depth at the radial direction of crystal rod could be increased with the increasing repetition rates.
A minimum laser threshold of 8.6 J is obtained with repetition frequency of 5 Hz and OC of 5% transmission, which is lower than the threshold of 10 at.% Er:YAP crystal (about 12.6 J) under the same condition. The decrease of laser threshold should be due to the co-doping of Pr3+ that greatly decreases the lifetime of the lower laser level. Nevertheless, the excessive co-doping of Pr3+ will also lead to the substantial decreasing of upper laser level lifetime, which is harmful to improve the laser output energy. Therefore, the doping concentrations of Er3+and Pr3+ still need a further optimization. Before that, the laser efficiency of Er,Pr:GGG (15.18%) has been improved by co-doping the Pr3+ into Er:GGG(13.84%) .
The laser performance of Er,Pr:YAP with an OC of 70% transmission is shown in Fig. 3(a). A maximum laser energy of 234.5 mJ is obtained with repetition frequency of 5 Hz, which value is smaller than the best result in 30% OC. But the low reflectivity of the OC is beneficial to reduce the energy flow in the cavity, so as to avoid damage the optical components . The polarization characteristic of Er,Pr:YAP laser with an OC of 70% transmission is also studied to obtain a higher power in the Q-switched experiment. As shown in Fig. 3(b), the polarization direction of the laser is parallel to the c axis of crystal. Therefore, the c axis of crystal is kept parallel to the polarization direction of Al2O3 polarizer (horizontal direction) during the Q-switched experiment, which can help decrease the wastage in the polarizing process.
3.2 Q-switched Er,Pr:YAP laser characteristics
Pulses with high peak power and short duration can be obtained when operating in the EO Q-switched mode. In general, the EO Q-switched laser was operated in the pulse-off mode to decrease the voltage applied on the LGS crystal . Our experimental setup is shown in Fig. 1. When a voltage of 5.6 kV is applied on the LGS crystal, the laser output energy reduces to 0 under a pump energy lower than 132 J, which means the EO Q-switch can stop completely the laser oscillation in the cavity, and a large amount of energy will store in the upper laser level. Therefore, the laser can work efficiently in the Q-switched mode. When the delay time between the trigger signal of the flash lamp and the trigger signal of the Q-switch is optimized to 310 µs, the highest peak power output is achieved. After optimizing the laser system mentioned above, a Q-switched giant pulse with 63.4 mJ pulse energy and 40 ns pulse width (see Fig. 4(b) for pulse shape) is obtained for pump energy of 132 J at 5 Hz. The energy conversion efficiency of static to dynamic Q-switched mode is 47%, corresponding to a peak power of 1.59 MW. As shown in Fig. 4(a), the pulse width decreases with the increase of the pump energy, and the output energy keeps increasing without saturation, which implies that the output energy can be further improved by increasing the pump energy. During the experiment, about an energy fluctuation of 2 mJ can be observed under the highest pump energy of 132 J, and the instability of the output pulse energy is about 3.1%. In our experiment, the pump energy isn’t further increased to avoid damaging the optical components.
3.3 M2 factor
After focusing laser beam through the lens with focal length 294.36 mm, the laser beam profiles of the Er,Pr:YAP laser are recorded by Pyroelectric Array camera (Ophir-Spiricon PY-III-HR) in free-running, static and dynamic Q-switched modes under the same pump energy of 105.84 J, and then the M2 factor and far-field divergence are determined through the hyperbolic fitting. Besides, the laser in three modes are attenuated to a same energy of about 140 µJ and then incident upon the Pyroelectric Array camera, which is beneficial to avoid the damage of camera and excluded the effect of energy intensity on the laser beam profiles. The results are shown in Fig. 5. The Mx2/My2 factors in the x and y directions are 8.36/8.11 for Er,Pr:YAP laser in free-running mode, 9.22/9.40 for static and 7.91/6.32 for dynamic Er,Pr:YAP laser in Q-switched modes, respectively. The far-field divergence Θx/Θy of Er,Pr:YAP laser in free-running, static and dynamic Q-switched modes are 9.91/9.89, 10.39/10.44 and 10.11/10.10 mrad, respectively. According to the experiment results, we can note that the beam quality of Er,Pr:YAP laser has relatively decrease after inserting the Al2O3 polarizer and LGS crystal in the static Q-switched mode, and then it also has certain improvement under dynamic Q-switched mode.
3.4 Laser spectrum of the Er,Pr:YAP crystal
The specific spectral compositions of Er,Pr:YAP laser under different pump energies and OCs are illustrated in Fig. 6(a), which are measured by a grating spectrometer (Omni-λ2005i, Zolix, China) with a slit width of 0.1 mm and a step width of 0.1 nm. The grating spectrometer (Omni-λ2005i, Zolix, China) has a focus length of 200 mm, equipped with a 300 g/mm grating, blazing wavelength located at 3000 nm. The slit width of entrance slit can be adjusted from 0.01-3 mm, the smallest step size can be adjusted to 0.01 nm. Unlike the four-wavelengths laser output of Er:YAP crystal , there are only two wavelengths laser output have been realized in Er,Pr:YAP crystal. The central wavelengths are located at 2713.2 ± 0.2 and 2732.0 ± 0.2 nm, corresponding to the full width at half maximum (FWHM) of 1.5 ± 0.1 nm and 1.6 ± 0.1 nm, respectively. The lasers with wavelengths of 2.79 and 2.92 µm are not obtained, but this two spectral lines usually have higher laser efficiency, which may be one of the reasons for the decreasing laser efficiency of Er,Pr:YAP crystal. It is worth noting that the thresholds of these two laser lines are different, the laser line located at 2713.2 nm possesses a lower threshold than that of 2732.0 nm, so the laser output can be realized in lower pump level, then with the increase of pump energy, the laser line output located at 2732.0 nm can be obtained and the intensity gradually increases. Besides, the laser thresholds (both 2713.2 and 2732.0 nm laser wavelength) of Er,Pr:YAP increase with the increase of transmittance of the output mirrors, which the phenomenon is particularly evident at the wavelength of 2713 nm. Just as shown in the Fig. 6, the emitting and intensity of the 2732.0 nm wavelength exceeds the wavelength of 2713.2 nm in a pump energy of 17.34 J at T=5%, but at T=70%, the 2732.0 nm wavelength does not realize laser output until the pump energy increases to 132.54 J.
Besides, the laser spectrum of Er,Pr:YAP laser in free-running, static and dynamic Q-switched modes under the same pump energy of 82.14 J are also measured with an OC of 70% transmittance, just as shown in Fig. 6(b). It is worth noting that the inset of LGS crystal and the Al2O3 medium polarizer, as well as the realization of dynamic Q-switched modes will not change the laser wavelength of Er,Pr:YAP laser. But these factors will introduce more energy loss and then cause the increase of laser threshold, which is the main reason for the decrease of intensity in the laser spectrum.
The laser performance of xenon lamp pumped Er,Pr:YAP crystal is studied in free-running and Q-switched modes, respectively. A maximum laser energy of 346 mJ is obtained with repetition frequency of 5 Hz and OC of 30% transmission in free-running mode. Compared with Er:YAP crystal, the minimum laser threshold is decreased effectively by co-doping deactivation ions Pr3+. The possible reasons of lower laser output energy are inapposite doping concentrations and only two laser output wavelengths with lower efficiency are obtained in the Er,Pr:YAP crystal. After optimizing the laser system, a Q-switched giant pulse with 63.4 mJ pulse energy and 40 ns pulse width is realized for a pump energy of 132 J and repetition frequency of 5 Hz. Besides, the laser beam quality factors are measured in free-running and Q-switched modes, respectively. Finally, The specific spectral compositions of Er,Pr:YAP laser are also studied in different conditions. The dual-wavelengths of 2713.2 and 2732.2 nm are realized in Er,Pr:YAP crystal. In summary, a xenon lamp pumped Electro-optically Q-switched 2.7 µm Er,Pr:YAP laser is successfully demonstrated for the first time and a peak power of 1.59 MW is obtained in this work. All the results indicate that the LGS Q-switched 2.7 µm Er,Pr:YAP laser is a promising candidate for the mid-infrared pulse laser device.
National Natural Science Foundation of China (51872290); National Key Research and Development Program of China (Grant Nos. 2016YFB1102301).
The authors declare that there are no conflicts of interest related to this article.
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